I am an avid Harry Potter fan. I’ve read all of the books (some more than once) and seen the movies. In general, I am a big fan of magic, which dates back to early childhood when I used to try to wiggle my nose just like Samantha Stevens in Bewitched. All that ever happened was a few sneezes. The sad truth is that magic only works on TV and for J. K. Rowling.
In residuals management, magic is not an ingredient to success — although transforming organic residuals into consumer friendly, usable soil amendments can be awe inspiring and almost magical! It is done by taking advantage of naturally occurring processes and optimizing conditions to accelerate these processes. This transformation occurs because of generally well-characterized reactions and biological processes.
So when a salesperson comes and offers a special inoculant or microbial tea to accelerate the composting process, be skeptical. Harry Potter’s magic often requires special potions; residuals management doesn’t. Microbes are behind natural transformation processes such as composting and anaerobic digestion. And microbes are a robust lot. They are ubiquitous. Every gram of soil contains about 6 million of them. Every person carries around 10 microbial cells for each human cell with 10,000 species of microbes in each person. In other words, pouring a special ‘microbial potion’ into a compost pile, even if it contains a billion critters, will go largely unnoticed by the existing occupants.
To aid organisms along, the one easy and cost-effective exception is to throw in a portion of seed inocula from the last batch of compost or digestate. This is a standard way to improve the composting process and enhance the rate of anaerobic digestion. A study examined start-up of a digester to process source separated MSW without inocula and found that things speeded up quickly with the addition of cattle manure (Maroun and El Fadel, 2007). Cattle manure is just chock full of anaerobic microorganisms that have been specially cultured in those rumens.
Mixing finished compost into a pile can provide the microbes necessary to reduce nitrous oxide emissions (Fukumoto et al., 2006). This last study compared addition of a “magical potion” (read cultured nitrite oxidizing bacteria) to just adding a shovel of the cured compost. Both provided the same number of bacteria and both worked equally well. The potion however, was a pain to make, taking at least seven weeks of lab time. Adding the finished compost in comparison just required a strong back or a loader.

Optimizing Conditions

Rather than focusing on potion addition, the key is to spend time optimizing conditions. These microbial transformations are all based on eating and energy. The microbes survive by eating the organics, which are full of carbon that has been transformed from gaseous carbon dioxide (CO2) to solid carbon compounds like carbohydrates and sugars via photosynthesis (see “Food Fight,” April 2012). This solid carbon contains energy and the microbes release that energy and satisfy their appetites by breaking the bonds and releasing the carbon back as CO2. A portion of the carbon they eat gets retained as microbial biomass or partially transformed carbon compounds (about 25% or so depending on what they are eating and the eating environment) with the rest released to the atmosphere.
This eating falls into two major groups: Aerobic (composting) and anaerobic (e.g., via anaerobic digestion). From a scientific perspective, aerobic digestion takes the electrons released from the carbon and stuffs them onto oxygen. With anaerobic digestion there isn’t any (or hardly any) oxygen, so electrons get stuffed onto a range of compounds including nitrogen, iron, sulfur and even carbon. All microbial transformations of organics revolve around these two processes — nothing else is out there.
Composting is based on having plenty of oxygen. And rather than relying on special elixirs for getting a compost pile to heat up, just make sure that the moisture content of the feedstock mix is between 40 and 60 percent. The other thing to remember is that microbes like a well balanced diet. Not too much roughage, not too much rich stuff. In other words, some types of carbon bonds are harder to break than others. Too much recalcitrant carbon and the pile won’t heat up. Too much carbon and not enough other nutrients and the microbes won’t have the vitamins needed to reproduce.
Anaerobic digestion is based on not enough oxygen. It is a slower and less efficient eating process in comparison to aerobic decomposition. This is good thing as it can produce methane, which is formed when microbes are desperate for a place to put electrons from eating carbon. Shoving these onto already reduced carbon will take a lot of energy — so the organisms that do this gain a lot less energy or calories from that pint of ice cream than you and I do. These are definitely skinny organisms. In addition, these microbes will also put electrons on sulfur instead of carbon whenever possible. Reduced sulfur compounds, a product of anaerobic digestion, are the inevitable and primary source of odor for this process.
As noted earlier, adding inocula from the last batch is the best way to make sure the right organisms are present to decompose the feedstocks. Controlling the conditions is also key. Here pH is a master variable. Anaerobic decomposition comes to a screeching halt at acidic pH. Like composting, anaerobic digestion also requires a balanced diet. Adding too much of a new food to a digester can cause indigestion. Instead, add new feedstocks slowly at first to let the microbes adjust (see “Picky Eaters,” March 2010).
Finally, beware of salespeople who claim they have a way to make all of the solids go away. While both composting and anaerobic digestion result in release of much of the initial feedstocks as CO2, there is always something left. For this I am eternally grateful as it is the leftovers from this eating process that give us such valuable soil amendments.
Decomposition of organics is a magical process. But the key to the magic is that it will happen if you just set the stage — no special ingredients required.
 
Sally Brown — Research Associate Professor at the University of Washington in Seattle — authors this regular column. Email Dr. Brown at slb@u.washington.edu.

References

Fukumoto, Y., K. Suzuki, T. Osada, K. Kuroda, D. Hanajima, T. Yasuda, and K. Haga. 2006. Reduction of nitrous oxide emission from pig manure composting by addition of nitrite-oxidizing bacteria. Environ. Sci. Tech., 40:6787-6791.
Maroun, R. and M. El Fadel. 2007. Start-up of anaerobic digestion of source-sorted organic municipal solid waste in the absence of classical inocula. Environ. Sci. Tech., 41:68-8-6814.